1. Field of the Disclosure
The disclosure generally relates to a method of characterizing particles according to the Coulter principle and, more specifically, to a method of characterizing large particles present among a mixture of predominantly uniform-sized smaller particles.
2. Brief Description of Related Technology
Liquid suspensions of sized particles are useful in many industries. These liquids often are of great benefit when the sized particles are homogenous, preferably in terms of size and weight. For example, chemical mechanical planarization is a liquid flow application traditionally used in glass polishing, and commonly used today in the manufacture of semiconductors. Chemical mechanical planarization (CMP) applications employ slurries to polish silicon wafers and provide uniform, scratch-free surfaces for fabrication of semiconductor devices. These slurries are often prepared in a central location, mixed, and then conveyed through distribution lines and dispensed to equipment suitable for polishing surfaces. These slurries are generally made up of polydisperse colloidal or fumed abrasive particles. Importantly, these slurries often contain very fine particles up to 0.2 micrometers (μm) in size. Larger size particles (e.g., 1 μm or greater) are often present in the slurries at a process point where the slurry is dispensed for polishing. These larger particles may have been formed as a result of shearing, agglomeration, or due to the presence of a foreign material in the slurry. The agglomerated slurry particles present during polishing are undesirable and can cause many defects in the wafer such as, for example, scratches due to deep indentation or non-uniform polishing due to differential polishing pressure. The presence of agglomerated particles can be minimized by carefully milling the fine particles, continuously mixing the slurry upstream of its distribution, maintaining a proper electrolyte balance, and employing filters at dispensation points to remove the agglomerates before the slurry is delivered to the wafer.
The desirability of homogeneously-sized particles with a certain, minimized concentration of agglomerates is certainly not limited to CMP applications. To the contrary, this desirability extends to various other applications. For example, in the pharmaceutical industry, agglomerates and disparately-sized particles are undesired in pharmaceutical preparations that must include accurate and reproducible doses of active substances. These particles can include proteins, which may tend to agglomerate when in solution. Agglomeration can affect the activity and stability of the proteins, therefore the pharmaceutical industry needs to know whether solutions of proteins include agglomerated proteins and the concentration of these agglomerated proteins. Furthermore, where these preparations are made for consumer use, consumer appeal is of greater concern, which means that the preparation may need to have uniform, aesthetically pleasing, and reproducible qualities. Another example where homogeneously-sized particles with a certain, minimized concentration of agglomerates are valued is in the manufacture of toner compositions used in laser printers and photocopy machines. Toner products that have significant concentrations of agglomerates or large particles can cause defects in the printed materials and can also damage printers and photocopy machines. As in the case of CMP slurries, the art relevant to the manufacture of pharmaceuticals and toner products has attempted to address the need to minimize large particles by improving milling and mixing procedures and employing filters.
But, despite attempts to address the need to characterize large particles in a solution predominantly composed of uniformly-sized fine (smaller) particles, there remain inefficiencies in the art. And those inefficiencies can affect manufacturing processes (e.g., semiconductor manufacturing and pharmaceutical manufacturing) in undesired ways. Accordingly, there remains a need to more efficiently detect and characterize large particles in suspensions predominantly composed of uniformly-sized fine (smaller) particles.